Polymers in which the elastomeric behavior does not depend on a chemical crosslinking of linear, highly polymeric molecules, but is caused by a physical interaction between specific parts of the molecules, have been widely used in industry for many years. These include, for example, block copolymers in which hard segment regions are embedded in a matrix of elastic segments composed of relatively long-chain blocks. The polymer compositions thus form two incompatible phases which do not separate fully because they are linked via the same polymer chain and their movement is thus restricted. The hard, that is to say rigid, regions thus effect a physical crosslinking and impart an elastomeric behavior to the polymers.
A comprehensive presentation on this subject is found in Thermoplastic Elastomers, Process Economics Program, Stanford Research Inst., Menlo Park, Calif., 1976.
In particular, polyurethanes containing polyether or polyester blocks have acquired importance as elastomers of this type. Thus, French Pat. No. 2,324,666 describes polyether-urethanes which contain units formed from polyether-diols, specific low-molecular weight diols and aromatic diisocyanates. French Pat. No. 2,313,410 described polyether-esters which contain urethane groups and units formed from polyethylene adipate, 1,4-butane diol and aromatic diisocyanates. Polymers to this type are normally no longer susceptible to crosslinking reactions.
A disadvantage of these polymers, termed "Thermolastics", can be seen in the fact that their dimensional stability under a high mechanical loading such as the action of compressive, enlongating or shearing forces, is not comparable to that of classical elastomers, i.e. chemically crosslinked elastomers such as vulcanized polyisoprene. Many fields of application have therefore been closed to the thermolastics.
Polyurethanes of a similar structure, but having terminal, polymerizable double bonds, are used for producing photopolymerizable recording materials such as printing plates and photoresists. In most cases these polyurethanes are viscous to highly viscous liquids, or soids. They are normally used in admixture with other components.
The essential constituents of such mixtures include a compound which can be polymerized by free radicals and which has terminal, ethylenically unsaturated groups; a binder, and an initiator which can be activated by radiation.
Polymerizable mixtures of this type are, e.g., disclosed in U.S. Pat. Nos. 2,760,863; 3,060,023 and 3,149,975. The polymerizable, ethylenically unsaturated compounds described in these texts are low-molecular weight and high-molecular weight compounds having terminal or side chain vinyl or vinylidene groups, particularly acrylic and methacrylic acid esters of low-molecular weight or high-molecular weight polyhydroxy compounds. In practice, polymerizable compounds which have hitherto gained acceptance are almost exclusively photopolymerizable materials based on such esters. Of these compounds, it is particularly the low-molecular weight compounds which have been preferentially employed in industry.
Such mixtures are used in reproduction technology for producing printing plates, photoresists (i.e. etch resists and electro-plating resists) and relief images which may be colored.
A particular group of mixtures of this type suitable for producing photoresists, is used in the form of a dry, photopolymerizable layer on a temporary support formed from a transparent, flexible plastic film. The layer is laminated under pressure and heat to a metal carrier such as a copper plate, which is to be modified image-wise; the layer is exposed while on the plate and developed to give a photoresist. Such materials and methods of processing are described in German Auslegeschrift No. 1,522,515 and German Offenlegungschrift No. 2,064,079.
The polymerizable compounds used in such mixtures are generally compounds which contain urethane groups and have terminal acrylic or methacrylic acid ester groups and which can also contain oligo-ether units. Mixtures which contain such oligomeric, polymerizable compounds and which generally exhibit particularly good flexibility are described, e.g, in German Offenlegungsschriften Nos. 2,822,189; 2,822,190 and 2,822,191, in German Auslegeschrift No. 2,115,373 and in U.S. Pat. Nos. 3,850,770 and 3,960,572.
Oligo-urethane compounds of this type also have been disclosed for producing synthetic elastomers having desirable properties, as described in British Pat. No. 1,441,108.